Re: [PATCH v3 2/2] drm/probe-helper: For DP, add 640x480 if all other modes are bad

[Doug Anderson <dianders@chromium.org>]

Hi,

On Wed, May 11, 2022 at 3:58 PM Douglas Anderson <dianders@chromium.org> wrote:
>
> As per Displayport spec section 5.2.1.2 ("Video Timing Format") says
> that all detachable sinks shall support 640x480 @60Hz as a fail safe
> mode.
>
> A DP compliance test expected us to utilize the above fact when all
> modes it presented to the DP source were not achievable. It presented
> only modes that would be achievable with more lanes and/or higher
> speeds than we had available and expected that when we couldn't do
> that then we'd fall back to 640x480 even though it didn't advertise
> this size.
>
> In order to pass the compliance test (and also support any users who
> might fall into a similar situation with their display), we need to
> add 640x480 into the list of modes. However, we don't want to add
> 640x480 all the time. Despite the fact that the DP spec says all sinks
> _shall support_ 640x480, they're not guaranteed to support it
> _well_. Continuing to read the spec you can see that the display is
> not required to really treat 640x480 equal to all the other modes. It
> doesn't need to scale or anything--just display the pixels somehow for
> failsafe purposes. It should also be noted that it's not hard to find
> a display hooked up via DisplayPort that _doesn't_ support 640x480 at
> all. The HP ZR30w screen I'm sitting in front of has a native DP port
> and doesn't work at 640x480. I also plugged in a tiny 800x480 HDMI
> display via a DP to HDMI adapter and that screen definitely doesn't
> support 640x480.
>
> As a compromise solution, let's only add the 640x480 mode if:
> * We're on DP.
> * All other modes have been pruned.
>
> This acknowledges that 640x480 might not be the best mode to use but,
> since sinks are _supposed_ to support it, we will at least fall back
> to it if there's nothing else.
>
> Note that we _don't_ add higher resolution modes like 1024x768 in this
> case. We only add those modes for a failed EDID read where we have no
> idea what's going on. In the case where we've pruned all modes then
> instead we only want 640x480 which is the only defined "Fail Safe"
> resolution.
>
> This patch originated in response to Kuogee Hsieh's patch [1].
>
> [1] https://lore.kernel.org/r/1650671124-14030-1-git-send-email-quic_khsieh@quicinc.com
>
> Signed-off-by: Douglas Anderson <dianders@chromium.org>
> Tested-by: Kuogee Hsieh <quic_khsieh@quicinc.com>
> Reviewed-by: Abhinav Kumar <quic_abhinavk@quicinc.com>
> Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
> ---
>
> Changes in v3:
> - Removed WARN_ON
>
> Changes in v2:
> - Two underscores for __drm_helper_update_and_validate().
> - Return err and use WARN_ON instead of returning a bool.
>
>  drivers/gpu/drm/drm_probe_helper.c | 27 ++++++++++++++++++++++-----
>  1 file changed, 22 insertions(+), 5 deletions(-)

Pushed to drm-misc-next:

e7c254d75d16 drm/probe-helper: For DP, add 640x480 if all other modes are bad